Method and apparatus for accessing channel in wlan system

ABSTRACT

The present invention relates to a wireless communication system, and more specifically, disclosed are a method and an apparatus for accessing a channel in a WLAN system. The method for accessing a channel from a station (STA) in a wireless communication system, according to one embodiment of the present invention, comprises the steps of: receiving from an access point (AP) a first frame including a traffic indication map (TIM) and a restricted access window (RAW) parameter set component; determining a RAW in which channel access of the STA is permitted, on the basis of the RAW parameter set (RPS) component; and transmitting a second frame to the AP from within the RAW that is determined, wherein the RPS component includes at least one RAW indication field, each of the at least one RAW allocation fields further includes information related to transmitting a third frame, which includes information on slot allocation, and wherein the third frame can be received by the STA from the AP at a specific time/location in accordance with the information related to transmitting the third frame.

This application is a continuation of U.S. application Ser. No.14/397,622 filed Oct. 28, 2014, which is a 35 USC §371 National Stageentry of International ApplicationNo. PCT/KR2013/003662 filed on Apr.29, 2013, and claims priority to U.S. Provisional Application Nos.61/639,877 filed on Apr. 28, 2012; 61/651,002 filed on May 24, 2012 and61/680,227 filed on Aug. 6, 2012, all of which are hereby incorporatedby reference in their entireties as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method and apparatus for accessing a channel ina WLAN system.

BACKGROUND ART

With recent development of information communication technologies, avariety of wireless communication technologies have been developed. Fromamong such technologies, WLAN is a technology that allows wirelessaccess to the Internet at home, in businesses, or in specific serviceproviding areas using a mobile terminal, such as a personal digitalassistant (PDA), a laptop computer, and a portable multimedia player(PMP), based on radio frequency technology.

In order to overcome limited communication speed, which has been pointedout as a weak point of WLAN, technical standards have recentlyintroduced a system capable of increasing the speed and reliability of anetwork while extending a coverage region of a wireless network. Forexample, IEEE 802.11n supports high throughput (HT) with a maximum dataprocessing speed of 540 Mbps. In addition, Multiple Input and MultipleOutput (MIMO) technology, which employs multiple antennas for both atransmitter and a receiver in order to minimize transmission errors andto optimize data rate, has been introduced.

DISCLOSURE Technical Problem

Machine-to-machine (M2M) communication technology has been discussed asa next generation communication technology. Technical standard tosupport M2M communications in the IEEE 802.11 WLAN system is also underdevelopment as IEEE 802.11ah. In M2M communications, a scenario in whichoccasional transmission/reception of a small amount of data at a lowspeed in an environment including a large number of devices may beconsidered.

Communication in the WLAN system is performed on a medium shared by alldevices. If the number of devices increases as in the case of M2Mcommunication, consumption of a lot of time for channel access of onedevice may deteriorate overall system performance and obstruct eachdevice from saving power.

An object of the present invention devised to solve the problem lies ina new channel access method for reducing time taken for channel accessand lowering power consumption of a device.

Objects of the present invention are not limited to the aforementionedobjects, and other objects of the present invention which are notmentioned above will become apparent to those having ordinary skill inthe art upon examination of the following description.

Technical Solution

The object of the present invention can be achieved by providing amethod for performing channel access in at least one station (STA) of awireless communication system, including receiving, from an accesspoint, a first frame containing a traffic indication map (TIM) elementand a restricted access window (RAW) parameter set element, determininga RAW allowing channel access of the STA based on the RAW parameter set(RPS) element, and transmitting a second frame to an access point (AP)within the determined RAW, wherein the RPS element includes at least oneRAW assignment field, wherein each of the at least one RAW assignmentfield contains information related to transmission of a third framecontaining information about assignment of slots, wherein the thirdframe is received from the AP by the STA at a specific time positionaccording to the information related to transmission of the third frame.

In another aspect of the present invention, provided herein is a station(STA) for performing channel access in a wireless communication system,including a transceiver, and a processor, wherein the processor isconfigured to receive, from an access point, a first frame containing atraffic indication map (TIM) element and a restricted access window(RAW) parameter set element using the transceiver, determine a RAWallowing channel access of the STA based on the RAW parameter set (RPS)element, and transmit a second frame to an access point (AP) within thedetermined RAW using the transceiver, wherein the RPS element includesat least one RAW assignment field, wherein each of the at least one RAWassignment field contains information related to transmission of a thirdframe containing information about assignment of slots, wherein thethird frame is received from the AP by the STA at a specific timeposition according to the information related to transmission of thethird frame.

Embodiments according to the above aspects of the present invention mayinclude the following details in common

The specific time position may be a time position for start of a nextRAW subsequent to the RAW having the second frame transmitted therein.

The STA may wake up and receive the third frame at the time position forstart of the next RAW.

The third frame may be a RAW announcement frame, the RAW announcementframe containing information about assignment of the slots in an RAW fortransmission of the third frame.

When the third frame is not transmitted by the AP, slot assignment inthe RAW may be determined based on an association identifier (AID) ofthe STA and the number of slots in the RAW.

Each of the at least one RAW assignment field may include a RAW groupfield, a RAW start time field and a RAW duration field.

The RAW group field may indicate association identifiers (AIDs) of STAsallowed to perform channel access within the RAW, wherein whether or notthe STAs belong to a group indicated by the RAW group field may bedetermined.

The RAW may include at least one slot, and each of the at least one RAWassignment field may include a slot duration field and a cross slotboundary field.

The slot duration field may indicate a duration of the at least one slothaving the same value within the RAW.

The cross slot boundary field may indicate whether or not transmissionby the STA is allowed to cross a slot boundary.

Each of the at least one RAW assignment field may include a fieldindicating whether or not the channel access is restricted to paged STAsonly.

The STA may operate in a doze state before a time, and switch to anawake state at the time, the channel access within the RAW being allowedat the time.

The first frame may be a beacon frame, and the second frame may be apower save (PS)-Poll frame or a trigger frame.

The second frame may be transmitted within the RAW based on enhanceddistributed channel access (EDCA).

The above general description and following detailed description of thepresent invention are exemplarily given to supplement the recitations inthe claims.

Advantageous Effects

According to one embodiment of the present invention, a new channelaccess method and apparatus for reducing time taken for channel accessand lowering power consumption of a device may be provided.

The effects that can be obtained from the present invention are notlimited to the aforementioned effects, and other effects may be clearlyunderstood by those skilled in the art from the descriptions givenbelow.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are intended to provide a furtherunderstanding of the present invention, illustrate various embodimentsof the present invention and together with the descriptions in thisspecification serve to explain the principle of the invention.

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system.

FIG. 5 illustrates a link setup process in a WLAN system.

FIG. 6 illustrates a backoff process.

FIG. 7 illustrates a hidden node and an exposed node.

FIG. 8 illustrates RTS and CTS.

FIG. 9 illustrates a power management operation.

FIGS. 10 to 12 illustrate operations of a station (STA) having receiveda TIM in detail.

FIG. 13 illustrates a group-based AID.

FIG. 14 illustrates the conventional TIM-based channel access method.

FIG. 15 illustrates the basic concept of a slotted channel accessmethod.

FIG. 16 illustrates an exemplary format of an RPS IE.

FIG. 17 illustrates configuration of a RAW according to one embodimentof the present invention.

FIG. 18 illustrates slotted channel access according to one embodimentof the present invention.

FIG. 19 illustrates slotted channel access according to anotherembodiment of the present invention.

FIG. 20 illustrates multicast/broadcast slot assignment in a RAWaccording to one embodiment of the present invention.

FIG. 21 illustrates multicast/broadcast slot assignment in a RAWaccording to another embodiment of the present invention.

FIG. 22 illustrates a channel access method according to one embodimentof the present invention.

FIG. 23 is a block diagram illustrating a radio frequency apparatusaccording to one embodiment of the present invention.

BEST MODE

mReference will now be made in detail to exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to present allembodiments that can be implemented according to the invention. Thefollowing detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The embodiments described below are constructed by combining elementsand features of the present invention in a predetermined form. Theelements or features may be considered selective unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequence of operations discussed in theembodiments of the present invention may be changed. Some elements orfeatures of one embodiment may also be included in another embodiment,or may be replaced by corresponding elements or features of anotherembodiment.

Specific terms are employed in the following description for betterunderstanding of the present invention. Such specific terms may takeother forms within the technical scope or spirit of the presentinvention.

In some cases, well-known structures and devices are omitted in order toavoid obscuring the concepts of the present invention and importantfunctions of the structures and devices may be mainly illustrated in theform of block diagrams. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of an Institute of Electrical andElectronics Engineers (IEEE) 802 system, a 3rd Generation PartnershipProject (3GPP) system, a 3GPP Long Term Evolution (LTE) system, anLTE-Advanced (LTE-A) system, and a 3GPP2 system, which are wirelessaccess systems. That is, steps or parts which are not described toclearly reveal the technical spirit of the present invention in theembodiments of the present invention may be supported by the abovedocuments. All terminology used herein may be supported by at least oneof the aforementioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies such as, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), and SC-FDMA (Single Carrier FrequencyDivision Multiple Access). CDMA may be embodied through a radiotechnology such as UTRA (Universal Terrestrial Radio Access) orCDMA2000. TDMA may be embodied through radio technologies such as GSM(Global System for Mobile communication)/GPRS (General Packet RadioService)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may beembodied through radio technologies such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, and E-UTRA (Evolved UTRA). For clarity, thefollowing description mainly focuses on IEEE 802.11 systems, buttechnical features of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by interaction between components. A Basic ServiceSet (BSS) may correspond to a basic component block in an IEEE 802.11LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and each of the BSSsincludes two STAs as members thereof (i.e., STA1 and STA2 are includedin BSS1, and STA3 and STA4 are included in BSS2). In FIG. 1, an ellipseindicating each BSS may be understood as a coverage area in which STAsincluded in the BSS maintain communication. This area may be referred toas a basic service area (BSA). If an STA moves out of the BSA, the STAcannot directly communicate with the other STAs within the BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an independent BSS(IBSS). For example, the IBSS may take a minimized form consisting oftwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. This type of LAN may be configuredwhen the LAN is necessary, rather than being prescheduled. This networkmay be referred to as an ad-hoc network.

Memberships of an STA in a BSS may be dynamically changed depending onwhether the STA is switched on or off and whether the STA enters orleaves the BSS area. The STA may use a synchronization process to jointhe BSS to be a member of the BSS. To access all services of a BSSinfrastructure, the STA should be associated with the BSS. Suchassociation may be dynamically established and may involve use of adistribution system service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a distribution system (DS), a distribution systemmedium (DSM), and an access point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be limited by physical layer(PHY) performance. In some cases, such limited distance may besufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network including a plurality of BSSs, rather than beingindependently present as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof the DSM. In this regard, a wireless medium (WM) and the DSM arelogically distinguished from each other in IEEE 802.11. Respectivelogical media are used for different purposes and are used by differentcomponents. According to IEEE 802.11, such media are not restricted toeither the same or different media. The flexibility of the IEEE 802.11LAN architecture (DS architecture or other network architectures) can beexplained by the fact that plural media are logically different fromeach other. That is, the IEEE 802.11 LAN architecture can be implementedin various manners and may be independently specified by a physicalproperty of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STA1 and STA4) to access the DS. Moreover, since all APs basicallycorrespond to STAs, all APs are addressable entities. An address used byan AP for communication on the WM need not be identical to an addressused by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. Once the controlledport is authenticated, data (or frames) may be transmitted to the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an extendedservice set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may beconstructed by a DS and BSSs. In the IEEE 802.11 system, this type ofnetwork is referred to as an ESS network. The ESS may correspond to aset of BSSs connected to one DS. However, the ESS does not include theDS. The ESS network is characterized in that the ESS network is viewedas an IBSS network in a logical link control (LLC) layer. STAs includedin the ESS may communicate with each other and mobile STAs are movabletransparently from one BSS to another BSS (within the same ESS) in LLC.

Regarding relative physical locations of the BSSs in FIG. 3, IEEE 802.11does not assume any arrangement, and all the following arrangements arepossible. BSSs may partially overlap and this positional arrangement isgenerally used to provide continuous coverage. In addition, the BSSs maynot be physically connected, and a distance between BSSs is notlogically limited. The BSSs may be located at the same physical positionand this positional arrangement may be adopted to provide redundancy.One (or at least one) IBSS or ESS network may be physically present inone space as one (or at least one) ESS network. This may correspond toan ESS network form taken in the case in which an ad-hoc networkoperates in a location where the ESS network is present, in the case inwhich IEEE 802.11 networks of different organizations physicallyoverlap, or in the case in which two or more different access andsecurity policies are needed at the same location.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system.FIG. 4 shows an exemplary infrastructure BSS including a DS.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices such as laptop computers or mobile phones whichare generally handled directly by users. In the example of FIG. 4, STA1, STA 3, and STA 4 correspond to the non-AP STAs and STA 2 and STA 5correspond to AP STAs.

In the following description, the non-AP STA may be called a terminal, awireless transmit/receive unit (WTRU), user equipment (UE), a mobilestation (MS), a mobile terminal, or a mobile subscriber station (MSS).The AP is a concept corresponding to a base station (BS), a Node-B, anevolved Node-B (e-NB), a base transceiver system (BTS), or a femto BS inother wireless communication fields.

Link Setup Process

FIG. 5 illustrates a general link setup process.

To set up a link with respect to the network and transmit/receive dataover the network, the STA should perform network discovery andauthentication, establish association, and perform an authenticationprocedure for security. The link setup process may also be referred toas a session initiation process or a session setup process. In addition,the discovery, authentication, association, and security setup steps inthe link setup process may be collectively called an association step ina general sense.

Hereinafter, an exemplary link setup process will be described withreference to FIG. 5.

In step S510, an STA may perform the network discovery operation. Thenetwork discovery operation may include a scanning operation of the STA.That is, the STA needs to search for an available network so as toaccess the network. The STA needs to identify a compatible networkbefore participating in a wireless network. Herein, the process ofidentifying a network contained in a specific region is referred to asscanning

The scanning operation is classified into active scanning and passivescanning.

FIG. 5 exemplarily shows the network discovery operation including theactive scanning process. In the case of active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, in order to move betweenchannels and search for nearby APs. A responder transmits a proberesponse frame to the STA having transmitted the probe request frame, inresponse to the probe request frame. Herein, the responder may be thelast STA that has transmitted a beacon frame in a BSS of the scannedchannel In the BSS, the AP transmits a beacon frame, and thus the APserves as the responder. In the IBSS, STAs within the IBSS transmit abeacon frame in rotation, and thus the responder is not fixed. Forexample, the STA that has transmitted the probe request frame on Channel#1 and has received the probe response frame on Channel #1 may storeBSS-related information that is contained in the received probe responseframe and move to the next channel (for example, Channel #2) to performscanning (i.e., transmission/reception of a probe request/response onChannel #2) in the same manner

Although not shown in FIG. 5, scanning may be carried out in the passivescanning manner In performing the passive scanning operation, an STA toperform scanning waits for a beacon frame while moving from one channelto another. The beacon frame, which is one of the management frames inIEEE 802.11, is periodically transmitted to inform of presence of awireless network and to allow the STA performing scanning to find awireless network and participate in the wireless network. In a BSS, theAP periodically transmits the beacon frame. In an IBSS, STAs of the IBSStransmit the beacon frame in rotation. When an STA performing scanningreceives a beacon frame, the STA stores information about the BSScontained in the beacon frame and moves to the next channel. In thismanner, the STA records beacon frame information received on eachchannel. The STA having received a beacon frame stores BSS-relatedinformation contained in the received beacon frame, and then moves tothe next channel and performs scanning in the same manner

In comparison between active scanning and passive scanning, activescanning is more advantageous than passive scanning in terms of delayand power consumption.

After the STA discovers the network, the STA may perform authenticationin step S520. This authentication process may be referred to as firstauthentication, which is clearly distinguished from the security setupoperation of step S540, which will be described later.

The authentication process may include transmitting, by the STA, anauthentication request frame to an AP and transmitting, by the AP, anauthentication response frame to the STA in response to theauthentication request frame. The authentication frame used intransmitting an authentication request/response may correspond to amanagement frame.

The authentication frame may contain information about an authenticationalgorithm number, an authentication transaction sequence number, astatus code, a challenge text, a robust security network (RSN), a finitecyclic group, etc. This information, which is an example of informationthat may be contained in the authentication request/response frame, maybe replaced with other information, or include additional information.

The STA may transmit an authentication request frame to the AP. The APmay determine whether to authenticate the STA on the basis of theinformation contained in the received authentication request frame. TheAP may provide an authentication result to the STA through theauthentication response frame.

After the STA is successfully authenticated, the association process maybe conducted in step S530. The association process may include the stepsof transmitting, by the STA, an association request frame to the AP andtransmitting, by the AP, an association response frame to the STA inresponse.

For example, the association request frame may include informationrelated to various capabilities, a beacon listening interval, a serviceset identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a traffic indication map(TIM) broadcast request, an interworking service capability, etc.

For example, the association response frame may include informationrelated to various capabilities, a status code, an association ID (AID),supported rates, an enhanced distributed channel access (EDCA) parameterset, a received channel power indicator (RCPI), a received signal tonoise indicator (RSNI), mobility domain, a timeout interval (associationcomeback time), an overlapping BSS scan parameter, a TIM broadcastresponse, a QoS map, etc.

The aforementioned information, which corresponds to some parts ofinformation which can be contained in the association request/responseframe, may be replaced with other information or include additionalinformation.

After the STA is successfully associated with the network, the securitysetup process may be performed in step S540. The security setup processof step S540 may be referred to as an authentication process based on arobust security network association (RSNA) request/response. Theauthentication process of step S520 may be referred to as a firstauthentication process, and the security setup process of step S540 maybe simply referred to as an authentication process.

The security setup process of step S540 may include, for example, aprocess of performing private key setup based on 4-way handshakingthrough an extensible authentication protocol over LAN (EAPOL) frame. Inaddition, the security setup process may be performed using anothersecurity scheme that is not defined in IEEE 802.11 standards.

Evolution of WLAN

In order to overcome a limit in WLAN communication speed, IEEE 802.11nhas recently been established as a communication standard. IEEE 802.11naims to increase network speed and reliability as well as to extendwireless network coverage. More specifically, IEEE 802.11n supports ahigh throughput (HT) of a maximum data processing speed of 540 Mbps, andis based on multiple input multiple output (MIMO) technology in whichmultiple antennas are used at both a transmitter and a receiver.

With widespread use of WLAN technology and diversification of WLANapplications, there has been a need for development of a new WLAN systemcapable of supporting higher throughput than a data processing speedsupported by IEEE 802.11n. The next generation WLAN system forsupporting very high throughput (VHT) is the next version (for example,IEEE 802.11ac) of the IEEE 802.11n WLAN system, and is one of IEEE802.11 WLAN systems recently proposed to support a data processing speedgreater than or equal to 1 Gbps at a MAC service access point (MAC SAP).

In order to efficiently utilize a radio frequency channel, the nextgeneration WLAN system supports a Multi User Multiple Input MultipleOutput (MU-MIMO) transmission scheme in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, technology for supporting WLAN system operations inwhitespace is under discussion. For example, a technology forintroducing the WLAN system in TV whitespace (TV WS) such as a frequencyband (e.g., a band between 54 MHz and 698 MHz) left idle due totransition from analog TV to digital TV has been discussed under theIEEE 802.11af standard. However, this is simply illustrative, and thewhitespace may be viewed as a licensed band which is primarily usable bya licensed user. The licensed user means a user who has permission touse the licensed band, and may also be referred to as a licensed device,a primary user, an incumbent user, or the like.

For example, an AP and/or STA operating in the whitespace (WS) shouldprovide a function of protecting the licensed user. For example, in thecase in which a licensed user such as a microphone is already using aspecific WS channel which is in a frequency band divided according to aregulation to have a specific bandwidth in the WS band, the AP and/orSTA are not allowed to use the frequency band corresponding to the WSchannel in order to protect the licensed user. In addition, the APand/or STA should stop using a frequency band for transmission and/orreception of a current frame when the licensed user uses this frequencyband.

Accordingly, the AP and/or STA need to pre-check whether use of aspecific frequency band within the WS band is possible, namely whether alicensed user is operating in the frequency band. Checking whether alicensed user is operating in the specific frequency band is referred toas spectrum sensing. An energy detection scheme, a signature detectionscheme and the like are utilized as the spectrum sensing mechanisms. TheAP and/or STA may determine that a licensed user is using the specificfrequency band if the intensity of a received signal exceeds apredetermined value, or when a DTV preamble is detected.

Machine-to-machine (M2M) communication technology has been discussed asa next generation communication technology. Technical standard IEEE802.11ah to support M2M communication in the IEEE 802.11 WLAN system isalso under development. M2M communication, which represents acommunication scheme involving one or more machines, may also bereferred to as machine type communication (MTC) or machine-to-machine(M2M) communication. Herein, the machine may represent an entity thatdoes not require direct manipulation from or intervention of a user. Forexample, not only a meter or vending machine equipped with a wirelesscommunication module, but also user equipment such as a smartphone whichis capable of performing communication by automatically accessing thenetwork without manipulation/intervention by the user may be an exampleof the machines. M2M communication may include device-to-device (D2D)communication and communication between a device and an applicationserver. As examples of communication between a device and an applicationserver there are; communication between a vending machine and anapplication server, communication between a Point of Sale (POS) deviceand an application server, and communication between an electric meter,a gas meter or a water meter and an application server. M2Mcommunication-based applications may include security, transportationand healthcare applications. Considering the characteristics of theaforementioned application examples, M2M communication needs to supportoccasional transmission/reception of a small amount of data at a lowspeed in an environment including a large number of devices.

Specifically, M2M communication needs to support a large number of STAs.While the current WLAN system assumes that one AP is associated with upto 2007 STAs, various methods to support other cases in which many moreSTAs (e.g., about 6000 STAs) are associated with one AP have beendiscussed regarding M2M communication. In addition, it is expected thatthere will be many applications to support/require a low transfer ratein M2M communication. In order to smoothly support many STAs, an STA inthe WLAN system may recognize presence or absence of data to betransmitted thereto on the basis of a traffic indication map (TIM), andseveral methods to reduce the bitmap size of the TIM have been underdiscussion. In addition, it is expected that there will be much trafficdata having a very long transmission/reception interval in M2Mcommunication. For example, in M2M communication, a very small amount ofdata such as electric/gas/water metering is required to be transmittedand received at long intervals (for example, every month). In addition,in M2M communication, an operation of an STA is performed according to acommand provided on downlink (i.e., a link from an AP to a non-AP STA),and as a result data is reported on uplink (i.e., a link from the non-APSTA to the AP). Accordingly, an improved communication scheme on uplinkfor transmission of major data is mainly handled in M2M communication.Further, an M2M STA usually operates using a battery and it is oftendifficult for a user to frequently charge the battery. Accordingly, itis required to ensure a long service life by minimizing batteryconsumption. Moreover, it is expected that it will be difficult for auser to directly manipulate the M2M STA in a specific situation, andtherefore the M2M STA is required to have a function of self-recovery.Accordingly, methods have been discussed to efficiently support the casein which a very small number of STAs have a data frame to receive fromthe AP during one beacon period while the number of STAs to beassociated with one AP increases in the WLAN system and to lower powerconsumption of the STAs.

As described above, WLAN technology is rapidly evolving, and not onlythe aforementioned exemplary techniques but also other techniques fordirect link setup, improvement of media streaming throughput, support ofhigh-speed and/or large-scale initial session setup, and support of anextended bandwidth and operation frequency are under development.

WLAN Operating Below 1 GHz (in sub-1GHz)

As described above, IEEE 802.11ah standard which takes M2M communicationinto consideration as use cases is under discussion. IEEE 802.11ahstandard may operate in an unlicensed band of frequencies below 1 GHz(sub-1GHz) except the TV white space band, and have even larger coverage(e.g., up to 1 km) than existing WLAN which mainly provides indoorcoverage. That is, when WLAN is used in a band of sub-1GHz operatingfrequencies (e.g., 700 MHz to 900 MHz) rather than at a frequency of 2.4GHz or 5 GHz at which WLAN has conventionally operated, coverage of anAP increases by about two to three times at the same transmission powerdue to the propagation characteristics of this band. In this case, alarge number of STAs per AP may be allowed to perform access. Use casesconsidered in IEEE 802.11ah standard are summarized in Table 1 below.

TABLE 1 Use Case 1: Sensors and meters 1a: Smart Grid-Meter to Pole 1c:Environmental/Agricultural Monitoring 1d: Industrial process sensors 1e:Healthcare 1f: Healthcare 1g: Home/Building Automation 1h: Home sensorsUse Case 2: Backhaul Sensor and meter data Backhaul aggregation ofsensors Backhaul aggregation of Industrial sensors Use Case 3: ExtendedRange Wi-Fi Outdoor extended range hotspot Outdoor Wi-Fi for cellulartraffic offloading

According to Use Case 1 in Table 1, various kinds of sensors/meters mayaccess an 802.11ah AP to perform M2M communication. Particularly, asmart grid allows up to 6000 sensors/meters to access one AP.

According to Use Case 2 in Table 1, the 802.11ah AP providing widecoverage serves as a backhaul link for other systems such as IEEE802.15.4g.

According to Use Case 3 in Table 1, outdoor extended range hotspotcommunication may be supported in the outdoor extended range includingextended home coverage, campus wide coverage, and shopping malls. Inaddition, in Use Case 3, the 802.11ah AP may serve to reduce overload ofcellular traffic by supporting traffic offloading of cellular mobilecommunication.

Configuration of a physical layer (PHY) for communication in the sub-1GHz band as described above may be implemented by applying 1/10down-clocking on the existing IEEE 802.11ac PHY. In this case,20/40/80/160/80+80 MHz channel bandwidths in 802.11ac may provide,through 1/10 down-clocking, 2/4/8/16/8+8 MHz channel bandwidths in thesub-1 GHz band. Thereby, a guard interval (GI) may increase from 0.8 μsby 10 times to 8 μs. In Table 2 below, throughput of the 802.11ac PHY iscompared with that of the sub-1 GHz PHY.

TABLE 2 IEEE 802.11ac PHY 1/10 down-clocked sub-1GH PHY ChannelBandwidth/Throughput Channel Bandwidth/Throughput 20 MHz/86.7 Mbps 2MHz/8.67 Mbps 40 MHz/200 Mbps 4 MHz/20 Mbps 80 MHz/433.3 Mbps 8MHz/43.33 Mbps 160 MHz/866.7 Mbps 16 MHz/86.67 Mbps 80 + 80 MHz/866.6Mbps 8 + 8 MHz/86.66 Mbps

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of mediumaccess control (MAC) is a Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) mechanism. The CSMA/CA mechanism, which is alsocalled a Distributed Coordination Function (DCF) of IEEE 802.11 MAC,basically employs a “listen before talk” access mechanism. In accordancewith this access mechanism, the AP and/or STA may perform Clear ChannelAssessment (CCA) of sensing a radio frequency channel or medium in apredetermined time interval (e.g., DCF Inter-Frame Space (DIFS)), priorto data transmission. When it is determined in the sensing that themedium is in the idle state, frame transmission begins through themedium. On the other hand, when it is sensed that the medium is in theoccupied state, the AP and/or STA does not start transmission, butestablishes a delay time (e.g., a random backoff period) for mediumaccess, and attempts to perform frame transmission after waiting duringthe period. Through application of a random backoff period, it isexpected that multiple STAs will attempt to start frame transmissionafter waiting for different times, resulting in minimized collision.

In addition, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). HCF is based on the DCF and the point coordinationfunction (PCF). PCF refers to a polling-based synchronous access schemein which polling is periodically executed to allow all reception APsand/or STAs to receive a data frame. In addition, the HCF includesenhanced distributed channel access (EDCA) and HCF controlled channelaccess (HCCA). EDCA is achieved when the access scheme provided tomultiple users by a provider is based on contention. HCCA is achieved inthe contention-free channel access scheme which employs the pollingmechanism. In addition, the HCF includes a medium access mechanism forimproving Quality of Service (QoS) of the WLAN, and may transmit QoSdata during both the contention period (CP) and the contention freeperiod (CFP).

FIG. 6 illustrates a backoff process.

Hereinafter, operations based on a random backoff period will bedescribed with reference to FIG. 6. If the medium is switched from theoccupied or busy state to the idle state, several STAs may attempt totransmit data (or frames). In a method to minimize collision, each STAselects a random backoff count, waits for a slot time corresponding tothe selected backoff count, and then attempts to start transmission. Therandom backoff count has a value of a pseudo-random integer, and may beset to a value in a range between 0 and CW. Herein, CW is a contentionwindow parameter value. Although the CW parameter is given CWmin as theinitial value, the initial value may be doubled if transmission fails(for example, if ACK of the transmission frame is not received). If theCW parameter value is CWmax, CWmax is maintained until data transmissionis successful, and at the same time data transmission may be attempted.If data transmission is successful, the CW parameter value is reset toCWmin. Preferably, the values of CW, CWmin, and CWmax are set to 2n-1(where n=0, 1, 2, . . . ).

Once the random backoff process begins, the STA continuously monitorsthe medium while counting down the backoff slot according to adetermined backoff count value. If the medium is monitored as being inthe occupied state, the STA stops the countdown and waits for apredetermined time. If the medium is in the idle state, the remainingcountdown resumes.

In the example shown in FIG. 6, if a packet for STA3 to transmit reachesMAC of STA3, STA3 may confirm that the medium is in the idle state inthe DIFS and immediately transmit a frame. In the meantime, the otherSTAs monitor the busy state of the medium, and operate in the standbymode. During operation of STA3, each of STA1, STA2, and STA5 may havedata to be transmitted. If the idle state of the medium is monitored,each of STA1, STA2, and STA5 waits for the DIFS time and then performscountdown of the backoff slot according to a random backoff count valuewhich they have selected. In the example shown in FIG. 6, STA2 selectsthe lowest backoff count value and STA1 selects the highest backoffcount value. That is, when the STA2 starts data transmission aftercompleting backoff counting, the residual backoff time of STA5 isshorter than the residual backoff time of STA1. Each of STA1 and STA5temporarily stops countdown and waits while STA2 occupies the medium.When occupancy by the STA2 is terminated and the medium returns to theidle state, each of STA1 and STA5 waits for a predetermined DIFS time,and restarts backoff counting. That is, after the residual backoff slotas long as the residual backoff time is counted down, frame transmissionmay start. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, STA4 may be givendata to be transmitted while STA2 occupies the medium. In this case,when the medium is in the idle state, STA4 may wait for the DIFS time,perform countdown according to the random backoff count value selectedby the STA4, and then start frame transmission. FIG. 6 exemplarilyillustrates a case in which the residual backoff time of STA5 is equalto the random backoff count value of STA4 by chance. In this case,collision may occur between STA4 and STA5. If collision occurs betweenSTA4 and STA5, neither STA4 nor STA5 receives ACK and accordingly datatransmission fails. In this case, each of STA4 and STA5 may double theCW value, select a random backoff count value and then performcountdown. Meanwhile, STA1 waits while the medium is in the occupiedstate due to transmission operation by STA4 and STA5. In this case, whenthe medium returns to the idle state, STA1 waits for the DIFS time, andthen starts frame transmission after lapse of the residual backoff time.

Sensing Operation of STA

As described above, the CSMA/CA mechanism includes not only physicalcarrier sensing through which the AP and/or STA directly sense themedium, but also virtual carrier sensing. The virtual carrier sensing isperformed to address some problems (such as a hidden node problem)encountered in medium access. In virtual carrier sensing, MAC of theWLAN system may use a network allocation vector (NAV). By means of theNAV value, the AP and/or STA which is using the medium or has authorityto use the medium indicates, for another AP and/or another STA, theremaining time before a time at which the medium becomes available.Accordingly, the NAV value may correspond to a reserved period duringwhich the medium is used by the AP and/or STA to transmit a frame.Access of an STA having received the NAV value may be prohibited ordeferred during the corresponding period. NAV may be set according to,for example, the value of the duration field in the MAC header of aframe.

A robust collision detection mechanism has been introduced to reduce theprobability of such collision. Hereinafter, this mechanism will bedescribed with reference to FIGS. 7 and 8. The actual carrier sensingrange may not be identical to the transmission range, but for simplicityof description, it will be assumed that the actual carrier sensing rangeis identical to the transmission range.

FIG. 7 illustrates a hidden node and an exposed node.

FIG. 7(a) exemplarily shows a hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted.Specifically, when STA C performs carrier sensing prior to transmissionof data to STA B, STA C may determine that the medium is in the idlestate even in a situation in which STA A is transmitting information toSTA B. This is because transmission by STA A (i.e., occupied medium) maynot be sensed at the location of STA C. In this case, collision mayoccur since STA B receives information of STA A and information of STA Csimultaneously. In this case, STA A may be considered a hidden node ofSTA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 13(b), STA C hasinformation to be transmitted to STA D in a situation in which STA B istransmitting data to STA A. In this case, STA C may perform carriersensing and determine that the medium is occupied due to transmission ofSTA B. Therefore, although STA C has information to be transmitted toSTA D, STA C should wait until the medium switches back to the idlestate since the occupied state of the medium is sensed. However, sinceSTA A is actually located out of the transmission range of STA C,transmission from STA C may not collide with transmission from STA B inview of STA A, and STA C unnecessarily waits until STA B stopstransmission. In this case, STA C may be viewed as an exposed node ofSTA B.

FIG. 8 illustrates RTS and CTS.

In order to efficiently use the collision avoidance mechanism in anexemplary situation as shown in FIG. 7, short-signaling packets such asRTS (request to send) and CTS (clear to send) may be used. RTS/CTSbetween two STAs may be overheard by nearby STA(s), such that the nearbySTA(s) may consider whether information is communicated between the twoSTAs. For example, if an STA to transmit data transmits an RTS frame toanother STA that is to receive data, the STA to receive data maytransmit a CTS frame to nearby STAs, thereby informing the nearby STAsthat the STA is about to receive data.

FIG. 8(a) exemplarily shows a method to solve the hidden node problem.The method assumes a situation in which both STA A and STA C attempt totransmit data to STA B. If STA A transmits RTS to STA B, STA B transmitsCTS to both STA A and STA C located around STA B. As a result, STA Cwaits until STA A and STA B stop data transmission, and thus collisionis avoided.

FIG. 8(b) exemplarily shows a method to solve the exposed node problem.STA C may overhear RTS/CTS transmission between STA A and STA B, therebydetermining that no collision will occur when it transmits data toanother STA (e.g., STA D). That is, STA B may transmit RTS to all thenearby STAs, and transmits CTS only to STA A which actually has data totransmit. Since STA C receives only the RTS, but fails to receive theCTS of STA A, STA C may recognize that STA A is located outside of thecarrier sensing range of STA C.

Power Management

As described above, STAs in the WLAN system should perform channelsensing before they perform transmission/reception operation.Persistently performing channel sensing causes persistent powerconsumption of the STA. There is not much difference in powerconsumption between the reception state and the transmission state, andcontinuous maintenance of the reception state may cause large load toSTAs which are provided with limited power (i.e., operated by abattery). Therefore, if an STA maintains the reception standby mode soas to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. To address thisproblem, the WLAN system supports a power management (PM) mode of theSTA.

The PM mode of the STA is classified into an active mode and a powersave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. When the STAis in the awake state, the STA may normally perform frametransmission/reception, channel scanning, or the like. On the otherhand, the STA in the PS mode operates by switching between the sleepstate (or doze state) and the awake state. The STA in the sleep stateoperates with minimum power and performs neither frametransmission/reception nor channel scanning.

As the time for which the STA operates in the sleep state increases,power consumption of the STA is reduced, and accordingly the STAoperation duration increases. However, since transmission or receptionof the frame is not allowed in the sleep state, the STA cannotunconditionally operate in the sleep state for a long time. When the STAoperating in the sleep state has a frame to transmit to the AP, it maybe switched to the awake state to transmit/receive the frame. On theother hand, when the AP has a frame to transmit to the STA which is inthe sleep state, the STA cannot receive the frame nor recognize thepresence of the frame. Accordingly, in order to recognize presence orabsence of a frame to be transmitted to the STA (or in order to receivethe frame if the frame is present), the STA may need to switch to theawake state according to specific periodicity.

FIG. 9 illustrates a power management operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at predetermined time intervals (S211, S212, S213, S214, S215and S216). The beacon frame includes a traffic indication map (TIM)information element. The TIM information element contains informationindicating that the AP 210 has buffered traffic for the STAs associatedwith the AP 210 and that a frame will be transmitted. The TIM elementincludes a TIM used to inform of a unicast frame and a delivery trafficindication map (DTIM) used to inform of a multicast or broadcast frame.

AP 210 may transmit a DTIM once per three transmissions of the beaconframe. STA1 220 and STA2 222 are STAs operating in the PS mode. Each ofSTA1 220 and STA2 222 may switch from the sleep state to the awake stateat every wakeup interval of a predetermined period to receive the TIMelement transmitted by the AP 210. Each STA may calculate a switchingtime to switch to the awake state, based on its own local clock. In theexample shown in FIG. 15, it is assumed that the clock of the STAcoincides with that of the AP.

For example, the predetermined wakeup interval may be set in such amanner that STA1 220 can switch to the awake state at every beaconinterval to receive the TIM element. Accordingly, when AP 210 transmitsthe beacon frame for the first time (S211), STA1 220 may switch to theawake state (S221). Thereby, STA1 220 may receive the beacon frame andacquire the TIM element. If the acquired TIM element indicates thatthere is a frame to be transmitted to STA1 220, STA1 220 may transmit apower save (PS)-Poll frame, which requests transmission of the frame, tothe AP 210 (S221 a). In response to the PS-Poll frame, the AP 210 maytransmit the frame to STA 1 220 (S231). After completing reception ofthe frame, STA1 220 is switched back to the sleep state and operates inthe sleep state.

When the AP 210 transmits the beacon frame for the second time, themedium is in the busy state in which the medium is accessed by anotherdevice, and accordingly the AP 210 may not transmit the beacon frame atthe correct beacon interval, but may transmit the beacon frame at adelayed time (S212). In this case, STA1 220 is switched to the awakestate in accordance with the beacon interval, but does not receive thebeacon frame whose transmission is delayed, and is thus switched back tothe sleep state (S222).

When the AP 210 thirdly transmits the beacon frame, the beacon frame mayinclude a TIM element set to a DTIM. However, since the medium is in thebusy state, the AP 210 transmits the beacon frame at a delayed time(S213). STA1 220 may be switched to the awake state in accordance withthe beacon interval and acquire the DTIM through the beacon frametransmitted by the AP 210. It is assumed that the DTIM acquired by STA1220 indicates that there is no frame to be transmitted to STA1 220, butthere is a frame for another STA. In this case, STA1 220 may confirmthat there is no frame to receive and switch back to the sleep state tooperate in the sleep state. After transmission of the beacon frame, theAP 210 transmits the frame to the corresponding STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). STA1 220 mayadjust the wakeup interval for reception of the TIM element since it hasfailed to acquire information indicating presence of buffered trafficfor STA1 220 through the previous two operations of reception of the TIMelement. Alternatively, provided that signaling information foradjustment of the value of the wakeup interval of STA1 220 is containedin the beacon frame transmitted by the AP 210, the wakeup interval valueof the STA1 220 may be adjusted. In this example, STA1 220 may be set tobe switched to the awake state once at every three beacon intervals toreceive a TIM element, rather than being set to be switched between theoperating states at every beacon interval. Therefore, when the AP 210fifthly transmits the beacon frame (S215) after fourth transmission ofthe beacon frame (S214), STA1 220 remains in the sleep state, and thuscannot acquire the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame (S216), STA1 220 may beswitched to the awake state and acquire the TIM element contained in thebeacon frame (S224). Since the TIM element is a DTIM indicating presenceof a broadcast frame, STA1 220 may receive the broadcast frametransmitted by the AP 210 without transmitting a PS-Poll frame to the AP210 (S234). In the meantime, the wakeup interval set for STA2 230 mayhave a longer period than the wakeup interval of STA1 220. Accordingly,STA2 230 is switched to the awake state at a time point (S215) when theAP 210 fifthly transmits the beacon frame, such that the STA2 230 mayreceive the TIM element (S241). STA2 230 may recognize presence of aframe to be transmitted thereto through the TIM element and transmit thePS-Poll frame to the AP 210 in order to request frame transmission (S241a). The AP 210 may transmit a frame to STA2 230 in response to thePS-Poll frame (S233).

In order to operate/manage the PS mode as shown in FIG. 9, the TIMelement includes a TIM indicating presence or absence of a frame to betransmitted to the STA or a DTIM indicating presence or absence of abroadcast/multicast frame. The DTIM may be implemented through fieldsetting for the TIM element.

FIGS. 10 to 12 illustrate operations of an STA having received a TIM indetail.

Referring to FIG. 10, an STA is switched from the sleep state to theawake state to receive the beacon frame including a TIM from the AP. TheSTA may recognize presence of buffered traffic to be transmitted theretoby interpreting the received TIM element. After the STA contends withother STAs to access the medium for PS-Poll frame transmission, the STAmay transmit a PS-Poll frame to the AP to request data frametransmission. The AP, upon receiving the PS-Poll frame transmitted fromthe STA, may transmit a data frame to the STA. The STA may receive thedata frame and transmit an ACK frame to the AP in response to thereceived data frame. Thereafter, the STA may switch back to the sleepstate.

As shown in FIG. 10, the AP may operate in a manner of immediateresponse in which the AP transmits the data frame when a predeterminedtime (e.g., a short inter-frame space (SIFS)) elapses after the APreceives the PS-Poll frame from the STA. However, the AP may operate ina manner of deferred response if the AP fails to prepare a data frame tobe transmitted to the STA for the SIFS time after receiving the PS-Pollframe, which will be described in detail with reference to FIG. 11.

In the example of FIG. 11, the operations of the STA of switching fromthe sleep state to the awake state, receiving a TIM from the AP, andtransmitting the PS-Poll frame to the AP through contention areidentical to those in the example of FIG. 10. If the AP having receivedthe PS-Poll frame fails to prepare a data frame for the SIFS time, theAP may transmit an ACK frame to the STA instead of transmitting the dataframe. If the data frame is prepared after transmission of the ACKframe, the AP may perform contention and transmit the data frame to theSTA. The STA may transmit the ACK frame indicating successful receptionof the data frame to the AP, and then be switched to the sleep state.

FIG. 12 shows an exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. The STAs mayrecognize, through the received DTIM, that a multicast/broadcast framewill be transmitted. After transmitting the beacon frame including theDTIM, the AP may immediately transmit data (i.e., a multicast/broadcastframe) without transmitting/receiving the PS-Poll frame. While the STAscontinue to maintain the awake state even after receiving the beaconframe including the DTIM, the STAs may receive data and then switch backto the sleep state after data reception is completed.

TIM Structure

In the operation and management method of the power save (PS) mode basedon the TIM (or DTIM) protocol illustrated in FIGS. 9 to 12, STAs maydetermine presence or absence of a data frame to be transmitted theretothrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan association identifier (AID) to be allocated when an STA isassociated with an AP.

The AID is used as a unique ID of each STA within a BSS. For example, inthe current WLAN system, an AID may be assigned a value between 1 and2007. In the currently defined WLAN system, 14 bits for the AID may beallocated to a frame transmitted by an AP and/or an STA. Although theAID may be assigned any value up to 16383, values from 2008 to 16383 areset as reserved values.

The TIM element according to legacy definition is inappropriate for M2Mapplication in which a large number of STAs (e.g., at least 2007 STAs)are associated with one AP. If the conventional TIM structure isextended without any change, the TIM bitmap size may excessivelyincrease. Accordingly, it may be impossible to support the extended TIMstructure using the legacy frame format, and the extended TIM structureis inappropriate for M2M communications in which application of a lowtransfer rate is considered. In addition, it is expected that the numberof STAs having a reception data frame during one beacon period is verysmall. Therefore, in view of the aforementioned exemplary application ofM2M communication, it is expected that a TIM bitmap will have a largesize with most bits set to zero (0) in many cases. Therefore, there is aneed for a technology capable of efficiently compressing a bitmap.

In the legacy bitmap compression technology, a series of Os is omittedfrom the front part of a bitmap to define an offset (or start point)value. However, compression efficiency is not high in the case in whichthe number of STAs including a buffered frame is small, but there is agreat difference between AID values of the STAs. For example, in thecase in which a frame to be transmitted only to STAs whose AIDs are setto 10 and 2000 is buffered, the length of the compressed bitmap is 1990,but all the parts of the bitmap other than both end parts are set tozero (0). If the number of STAs associated with one AP is small,inefficiency of bitmap compression may not be a serious problem.However, if the number of STAs associated with one AP increases, suchinefficiency may deteriorate overall system performance.

In order to address this issue, AIDs may be divided into a plurality ofgroups such that data can be more efficiently transmitted with the AIDs.A designated group ID (GID) is allocated to each group. Hereinafter,AIDs allocated on the group basis will be described with reference toFIG. 20.

FIG. 13(a) is a diagram illustrating an exemplary AID allocated on thegroup basis. In FIG. 13(a), some bits located at the front part of theAID bitmap may be used to indicate a group ID (GID). For example, thefirst two bits of an AID bitmap may be used to designate four GIDs. Ifthe total length of the AID bitmap is N bits, the first two bits (B1 andB2) may represent a GID of a corresponding AID.

FIG. 13(b) is a diagram illustrating another exemplary AID allocated onthe group basis. In FIG. 13(b), a GID may be allocated according to theposition of an AID. In this case, AIDs having the same GID may berepresented by an offset and a length value. For example, if GID 1 isdenoted by an offset A and a length B, this means that AIDs A to A+B−1on a bitmap are set to GID 1. For example, FIG. 13(b) assumes that AIDs1 to N4 are divided into four groups. In this case, AIDs belonging toGID 1 are denoted by 1 to N1, and may be represented by an offset of 1and a length of N1. AIDs belonging to GID 2 may be represented by anoffset of N1+1 and a length of N2−N1+1, AIDs belonging to GID 3 may berepresented by an offset of N2+1 and a length of N3−N2+1, and AIDsbelonging to GID 4 may be represented by an offset of N3+1 and a lengthof N4−N3+1.

If AIDs allocated on the group basis are introduced, channel access maybe allowed in different time intervals according to GIDs. Thereby, theproblem of lack of TIM elements for a large number of STAs may be solvedand at the same time data transmission/reception may be efficientlyperformed. For example, in a specific time interval, channel access isallowed only for STA(s) corresponding to a specific group, and channelaccess of the remaining STA(s) may be restricted. A predetermined timeinterval in which only specific STA(s) are allowed to perform channelaccess may be referred to as a restricted access window (RAW).

Hereinafter, channel access based on GIDs will be described withreference to FIG. 13(c). FIG. 13(c) illustrates an exemplary channelaccess mechanism according to beacon intervals with AIDs divided intothree groups. A first beacon interval (or a first RAW) is an interval inwhich channel access of an STA corresponding to an AID belonging to GID1 is allowed, and channel access of STAs belonging to the other GIDs isnot allowed. To implement this mechanism, a TIM element used only forAIDs corresponding to GID 1 is contained in a first beacon frame. A TIMelement used only for AIDs corresponding to GID 2 is contained in asecond beacon frame. Accordingly, channel access is allowed only for anSTA corresponding to the AIDs belonging to GID 2 in a second beaconinterval (or a second RAW). A TIM element used only for AIDscorresponding to GID 3 is contained in a third beacon frame.Accordingly, channel access is allowed only for an STA corresponding tothe AIDs belonging to GID 3 in a third beacon interval (or a third RAW).A TIM element used only for AIDs corresponding to GID 1 is contained ina fourth beacon frame. Accordingly, channel access is allowed only foran STA corresponding to the AIDs belonging to GID 1 in a fourth beaconinterval (or a fourth RAW). Thereafter, only channel access of an STAcorresponding to a specific group indicated by the TIM contained in acorresponding beacon frame may be allowed in each of beacon intervalssubsequent to the fifth beacon interval (or in each of RAWs subsequentto the fifth RAW).

While FIG. 13(c) exemplarily shows a case in which the order of allowedGIDs is periodic or cyclic according to the beacon intervals,embodiments of the present invention are not limited thereto. That is,only AID(s) belonging to specific GID(s) may be contained in a TIMelement, such that only channel access of STA(s) corresponding to thespecific AID(s) is allowed in a specific time interval (for example, aspecific RAW), and channel access of the other STA(s) is not allowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, the entiretyof an AID space may be divided into a plurality of blocks, and onlySTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having a value other than ‘0’ may be allowed to perform channelaccess. Thereby, a large-sized TIM is divided into small-sizedblocks/groups, an STA can easily maintain TIM information, andblocks/groups may be easily managed according to class, QoS or usage ofthe STA. While FIG. 13 exemplarily shows a 2-level hierarchy, ahierarchical TIM structure comprised of two or more levels may beconfigured. For example, the whole AID space may be divided into aplurality of page groups, each page group may be divided into aplurality of blocks, and each block may be divided into a plurality ofsub-blocks. In this case, an extended version of the example of FIG.13(a) may be configured such that first N1 bits in an AID bitmaprepresent a page ID (PID), the next N2 bits represent a block ID, thenext N3 bits represent a sub-block ID, and the remaining bits representthe position of STA bits within a sub-block.

Various schemes for dividing STAs (or AIDs allocated to the STAs) intopredetermined hierarchical group units and managing the same may beapplied to the examples of the present invention disclosed below.However, the group-based AID allocation schemes are not limited to theseexamples.

U-APSD Mechanism

According to the unscheduled-automatic power save delivery (U-APSD)mechanism, in order to use a U-APSD service period (SP), an STA mayinform an AP of a requested transmission duration, and the AP maytransmit a frame to the STA during the SP. According to the U-APSDmechanism, the STA may receive multiple PSDUs from the AP at the sametime within its own SP.

The STA may recognize through the TIM element of the beacon that the APhas data to transmit to the STA. Thereafter, the STA may transmit atrigger frame to the AP. Thereby, the STA may inform the AP that theservice period (SP) of the STA has started, and request that the APtransmit the data. The AP may transmit ACK to the STA in response to thetrigger frame. Thereafter, the AP may transmit RTS to the STA throughcontention, receive a CTS frame from the STA, and then transmit the datato the STA. Herein, the data transmitted from the AP may include atleast one data frame. When the AP transmits the last data frame with theEOSP (End Of Service Period) field of the data frame set to 1, the STAmay recognize this and end the SP. Thereby, the STA may transmit ACKindicating successful reception of the data to the AP. According to theU-APSD mechanism described above, the STA is allowed to start its own SPand receive data when it desires and to receive multiple data frameswithin one SP. Accordingly, efficient data reception may be possible.

PPDU Frame Format

A PPDU (Physical Layer Convergence Protocol (PLCP) Packet Data Unit)frame format may include a STF (Short Training Field), an LTF (LongTraining Field), an SIG (SIGNAL) field, and a data field. The most basicPPDU frame format (e.g., a non-HT (High Throughput) PPDU frame format)may consist of an L-STF (Legacy-STF), an L-LTF (Legacy-LTF), an SIGfield, and a data field. In addition, depending on the type of a PPDUframe format (e.g., an HT-mixed format PPDU, an HT-greenfield formatPPDU, a VHT (Very High Throughput) PPDU, etc.), additional (or anothertype) STF, LTF, and SIG field may be included between the SIG field andthe data field.

The STF is a signal for signal detection, automatic gain control (AGC),diversity selection, precise time synchronization, and the like, and theLTF is a signal for channel estimation, frequency error estimation, andthe like. A combination of the STF and the LTF may be referred to as aPLCP preamble. The PLCP preamble may be a single for channel estimationand synchronization of an OFDM physical layer.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may contain information about data demodulation and coding rate.The LENGTH field may contain information about the length of data.Additionally, the SIG field may include a parity bit and an SIG TAILbit.

The data field may include a SERVICE field, a PSDU (PLCP Service DataUnit), a PPDU TAIL bit. When necessary, the data field may also includea padding bit. Some bits of the SERVICE field may be used to synchronizea descrambler of a receiver. The PSDU corresponds to a MAC PDU definedin the MAC layer, and may contain data produced/used in a higher layer.The PPDU TAIL bit may be used to return the state of an encoder set to0. The padding bit may be used to adjust the length of the data field ina predetermined unit.

A MAC PDU is defined according to various MAC frame formats, and a basicMAC frame includes a MAC header, a frame body, and an FCS (Frame CheckSequence). The MAC frame may be configured by the MAC PDU andtransmitted/received through a PSDU of the data part of a PPDU frameformat.

A null-data packet (NDP) frame format represents a frame format thatdoes not include a data packet. That is, an NDP frame includes the PLCPheader part (i.e., an STF, an LTF and an SIG field) of a typical PPDUformat, but does not include the other part (i.e., the data field) ofthe typical PPDU format. The NDP frame may be referred to as a shortframe format.

Slotted Channel Access Method

FIG. 14 illustrates the conventional TIM-based channel access method.

In FIG. 14, an STA corresponding to a bit set to 1 in a TIM elementcontained in a beacon frame may recognize presence of data to betransmitted thereto in a beacon interval, and may accordingly transmit aPS-Poll frame or a trigger frame to an AP. In the example of FIG. 14, itis assumed that a large number of STAs (e.g., at least 2007 STAs) areassociated with one AP (as in, for example, an outdoor smart gridnetwork). Herein, if n bits are set to 1 in the TIM element, n STAs(i.e., STA 1, STA 2, . . . , STA n) attempt to transmits a PS-Poll frameor a trigger frame to the AP in a short time interval after transmissionof the beacon frame.

In this case, if many STAs are present at the boundary portion of thecoverage of the AP, uplink transmission of an STA may be hidden from theother STAs. Moreover, if a large number of bits of the TIM element areset to 1, and transmission of PS-Poll frames or trigger frames from alarge number of STAs is performed in the short time interval afterbeacon frame, transmission collision between STAs may increase due tothe hidden node problem.

To solve this problem, the present invention proposes a slotted channelaccess method. Basically, the present invention proposes that a specifictime interval (e.g., a RAW) allowing uplink channel access of a smallernumber of STAs be set, or attempts of uplink channel access by a largenumber of STAs be distributed in a wide time interval in order to reducecollision and improve network performance.

FIG. 15 illustrates the basic concept of a slotted channel accessmethod.

The AP may distribute information about an AID segment to STAs through aDTIM announcement and a TIM announcement subsequent to the DTIMannouncement. A whole TIM bitmap may be divided into one or more segmentblocks, and may be configured by a combination of one or more TIMelements. That is, segment blocks may correspond to a part of the wholeTIM bitmap. AID segment information contained in the DTIM announcementor the TIM announcement may include, for example, a segment block offseta segment block range, a TIM for an AID segment, and information aboutthe duration of a RAW. The segment block offset may be a start positionof the AID segment, and the segment block range may represent the lengththereof. Thereby, only STAs (i.e., STAs having an AID included in theAID segment) covered by the AID segment are allowed to access a channelwithin a RAW immediately after the DTIM or TIM announcement.

A RAW may be divided into one or more time slots. The slot duration maybe differently set for each RAW. In the case in which one RAW includes aplurality of slots, the duration of the slots may be set to the samevalue. The information about the slot duration for each RAW may becontained in a beacon frame. An STA in the doze mode may wake up at atarget beacon transmission time (TBTT) and listen to a beacon frame,thereby acquiring slot duration information in the corresponding RAW.

In this manner, an STA corresponding to an AID segment provided througha DTIM or TIM announcement may recognize that channel access thereof isallowed in a RAW immediately after the DTIM or TIM and also recognize,from the slot duration information, the slot duration in the RAW. If theSTA further recognizes information about a RAW duration, it may infer ordetermine the number of slots included in the RAW from the slot durationinformation and the RAW duration information.

In this case, the STA may determine the position of a slot at which theSTA needs to perform channel access (or channel access is allowed)within the RAW, based on the AID bit position thereof. The STA mayacquire the AID bit position thereof from a specific information element(IE). Hereinafter, the IE will be referred to as a RAW parameter set(RPS) IE or a grouping parameter set (GrPS) IE, which represents a setof parameters which are needed for medium access restrictively allowedonly for a group of STAs.

FIG. 16 illustrates an exemplary format of an RPS IE.

The element ID field may be set to a value indicating that an IE is anRPS IE.

The Length field may be set to a value indicating the length of thefields following the Length field. The number of subsequent RAW fields(or RAW assignment fields) may be determined according to the value ofthe Length field.

N RAW fields (or RAW assignment fields) may be included in an RPS IE,and each RAW field includes parameters for one RAW.

Hereinafter, a description will be given of subfields included in a RAWfield shown in FIG. 16 with reference to FIG. 17.

FIG. 17 illustrates configuration of a RAW according to one embodimentof the present invention.

The Group ID field of FIG. 16 includes a segment bitmap or a blockbitmap, and provides identification information about a group for whichaccess is restrictively allowed within a corresponding RAW interval.That is, the Group ID field may contain information specifying an AIDsegment block (e.g., a start index, a length, an end index and the likeof the AID segment block). In this sense, the Group ID field may bereferred to as a RAW group field.

The RAW Start Time field of FIG. 16 may contain information about thestart time at which medium access of an STA group is allowed. The RAWstart time may be represented as a difference (or a duration) betweenthe end time of beacon transmission and the time at which a RAW starts,and the unit thereof may be TU (time unit). TU may be configured inmicroseconds (us). For example, TU may be defined as 1024 μs. If RAWStart Time is set to 0, the RAW may start immediately after the beaconframe ends, as shown in FIG. 17.

The RAW Duration field in FIG. 16 may contain information about thelength of time (i.e., the duration) for which medium access of an STAgroup is allowed. The RAW duration corresponds to a difference betweenthe RAW start time and the RAW end time, and the unit thereof may be TU.

The RAW Slot Duration field in FIG. 16 may contain information about thelength of time (i.e., the duration) of each of channel access slotsincluded in a RAW. As described above, each RAW may include a singleslot, or may include a plurality of time slots. In the case in whicheach RAW includes a plurality of time slots, the durations of the slotsincluded in a RAW have the same value. FIG. 17 shows a case in which sixslots are defined within one RAW duration, and the durations of the sixslots are set to the same value.

The RAW Slot Boundary field in FIG. 16 may be set to a value indicatingwhether or not a transmission opportunity (TXOP) or transmission withinthe TXOP is allowed to extend across (or crosses) a slot boundary. Theslot boundary refers to a time which serves as a reference fordistinguishing consecutive slots from each other as shown in FIG. 17. Inthis sense, the RAW Slot Boundary field may be referred to as a crossslot boundary field.

If a TXOP (or transmission within the TXOP) is not allowed to cross aslot boundary, the TXOP (or transmission within the TXOP) should endbefore the slot boundary. For example, in FIG. 17, an STA that attemptschannel access (namely, transmitting an uplink frame (a PS-Poll ortrigger frame)) in the first slot may receive data from the AP through adownlink frame and transmit an ACK frame to the AP in response to thedata. In the case in which the TXOP (or transmission within the TXOP) isnot allowed to cross a slot boundary, transmission of the ACK frameshould be completed within the corresponding slot. In addition, the APmay inform of whether the above TXOP rule (i.e., a TXOP (or transmissionin the TXOP) is not allowed to cross a slot boundary) is applied to eachRAW. If such TXOP rule is applied, the STA may not wait as long as theprobe delay when it wakes up on the slot boundary.

The RAW Slot AID field in FIG. 16 may be set to a value indicatingwhether or not channel access allowed only for an STA having a bitcorresponding to the AID of the STA set to 1 in the TIM element. Thatis, the RAW Slot AID field may indicate whether channel access (i.e.,transmission of an uplink frame) is allowed only for an STAcorresponding to an AID for which the bit value is set to 1 in the TIMbitmap (namely, a paged STA), or is allowed regardless of whether or notthe bit value is set to 1 in the TIM bitmap (namely, for paged STAs andunpaged STAs together). In this sense, the RAW Slot AID field may bereferred to as an Access Restricted to Paged STAs Only field.

The fields included in the GrPS IE or the RPS IE in FIG. 16 are simplyillustrative. A field configured in a different form and includingsubstantially the same information as the fields described above is alsowithin the scope of the present invention. In addition, the format ofthe proposed GrPS IE or RPS IE is not limited to the fields shown inFIG. 16. The format may include only some of the fields shown in FIG.16, or may further include other fields which are not shown in FIG. 16.

The GrPS IE or RPS IE described above with reference to FIG. 16 may betransmitted through a beacon frame, a probe response frame, or the like.When the GrPS IE or RPS IE is transmitted through a beacon frame, theGrPS IE or RPS IE may be broadcast by the AP. When the GrPS IE or RPS IEis transmitted through a probe response frame, unicast of the GrPS IE orRPS IE may be performed by the AP.

Slot Assignment

An STA may operate in the doze (or sleep) state until a channel accessslot assigned to the STA arrives. The STA may wake up on a slot boundaryof the channel access slot with which the STA is assigned and startchannel access in an EDCA manner (i.e., in a contention manner).

In this regard, which slot is assigned to each STA may be determined asfollows.

A channel access slot for an STA may be basically determined by modulooperation of the total number of slots of a corresponding RAW and theAID of the STA. For example, an index (i_(slot)) of a slot in which theSTA is allowed to start accessing a channel may be determined based onthe following equation.

i _(slot) =f(AID)mod N _(RAW)   Equation 1

In Equation 1, f(AID) has a value determined based on the AID of theSTA. For example, f(AID) may be defined such that the value of the AIDis used or only some bits of the AID are used.

In Equation 1, N_(RAW) denotes the total number of slots of a RAW.N_(RAW) may be calculated according to N_(RAW)=T_(RAW)/T_(slot). Herein,T_(RAW) has a RAW duration value, and T_(slot) has a slot durationvalue.

In Equation 1, ‘mod’ represents modulo operation. A mod B stands for aremainder of division of A by B. A mod B may be expressed as A % B.

In Equation 1, a full AID of an STA may be used for f(AID).Alternatively, a partial AID may be used for f(AID) in place of the AID.Partial AID is a non-unique identifier of the STA, and may be determinedby a hashing function using a part of the bits of the AID.

In the case in which Partial AID is used in calculating the slotassignment, a plurality of STAs (e.g., STAs having consecutive AIDvalues) may be assigned so as to use the same channel access slot. Forexample, in Equation 1, f(AID) may be defined as being determined basedon AID[a:b]. Herein, AID[a:b] represents Bit[a] to Bit[b] of the AIDwhich is a binary number. The value of a or b may be provided to eachslot by the AP.

For example, suppose that slot assignment is determined using AID[3:12].AID[3:12] represents Bit3 to Bit12 of AID having all 14 bits (from Bit0to Bit13). In this case, regardless of the values of Bit0, Bit1, Bit2and Bit13 of AID, STAs for which Bit3 to Bit12 of AID are set to thesame value may be allowed to perform channel access in the slot.

Alternatively, in the case in which RAWs are restrictively assigned toSTAs having AID corresponding to a bit having a bit value of 1 in theTIM element (namely, paged STAs) as shown in FIG. 20, which will bedescribed later, f(AID) in Equation 1 may be determined based on theposition index of the AID bit in the TIM element. That is, in an exampleas illustrated in FIG. 20, when four bits (i.e., the first, third, sixthand ninth bits) are set to 1 in the TIM bitmap, the position index ofAID1 corresponding to the first bit may be determined to be 1, theposition index of AID3 corresponding to the third bit may be determinedto be 2, the position index of AID6 corresponding to the sixth bit maybe determined to be 3, and the position index of AID9 corresponding tothe ninth bit may be determined to be 4. That is, when AIDs having a bitvalue of 1 in the TIM element are arranged in ascending order, the ordervalues thereof may correspond to the position indexes thereof.Accordingly, an STA having AID1 may be assigned the first slot in theRAW, another STA having AID3 may be assigned the second slot in the RAW,another STA having AID6 may be assigned the third slot in the RAW, andthe other STA having AID9 may be assigned the third slot in the RAW.

On the other hand, in the case in which f(AID) is defined as using AIDs(or Partial AIDs) of STAs, f(AID) may use AIDs when RAWs arerestrictively unassigned to STAs (e.g., paged STAs) having AIDscorresponding to bits set to 1 in the bitmap of the TIM element. Thatis, in the case in which channel access in a RAW is allowed for any STAs(e.g., all STAs regardless of whether or not the STAs are paged STAs),which slots in the RAW to be assigned to the STAs may be determinedbased on the AIDs of the STAs.

As described above, information about slot assignment may beadditionally contained (in the form of, for example, a slot assignmentfield) in the GrPS or RPS IE of FIG. 16.

Examples of Slotted Channel Access

FIG. 18 illustrates slotted channel access according to one embodimentof the present invention.

In the example of FIG. 18, it is assumed that the GrPS or RPS IE forRAW1 indicates that only STAs satisfying the following conditions areallowed to perform channel access in RAW1.

RAW Slot AID field: This field indicates that restriction is appliedaccording to bit values corresponding to AIDs of STAs in a TIM element(namely, only channel access of STAs whose AID bit values are set to 1in the TIM element (i.e., paged STAs) are allowed). In the example ofFIG. 18, STAs having AIDs corresponding to the first, third, sixth andninth bits in the TIM bitmap are allowed to access a channel in RAW 1.

RAW Slot Duration field: This field is set to T_(s1) (whereinT_(s1)=Length of PS-Poll frame+SIFS+Length of ACK frame, or Ts1=Lengthof Null Data Trigger frame+SIFS+Length of ACK frame).

RAW Slot Boundary field: This field indicates that a TXOP (ortransmission within the TXOP) is not allowed to cross a slot boundary.

With the configurations as above, RAW1 of FIG. 18 may be used only for aPS-Poll or null-data trigger frame.

In the example of FIG. 18, it is assumed that the GrPS or RPS IE forRAW2 indicates that only STAs satisfying the following conditions areallowed to perform channel access in RAW2.

RAW Slot AID field: This field indicates that restriction is appliedaccording to bit values corresponding to AIDs of STAs in a TIM element(namely, only channel access of STAs whose AID bit values are set to 1in the TIM element (i.e., paged STAs) are allowed). In the example ofFIG. 18, STAs having AIDs corresponding to the first, third, sixth andninth bits in the TIM bitmap are allowed to access a channel in RAW2.

RAW Slot Duration field: This field is set to T_(s2) (whereinT_(s2)≧Length of data frame+SIFS+Length of ACK frame).

RAW Slot Boundary field: This field indicates that a TXOP (ortransmission within the TXOP) is not allowed to cross a slot boundary.

With the configurations as above, RAW2 of FIG. 18 may be used by the APto transmit a data frame to STAs having AIDs corresponding to bits inthe TIM bitmap which have 1 as bit values thereof.

FIG. 19 illustrates slotted channel access according to anotherembodiment of the present invention.

In the example of FIG. 19, it is assumed that the GrPS or RPS IE forRAW1 indicates that only STAs satisfying the following conditions areallowed to perform channel access in RAW1.

RAW Slot AID field: This field indicates that restriction according tobit values corresponding to AIDs of STAs in a TIM element is not applied(namely, channel access of all STAs is allowed in RAW1 regardless ofwhether or not the AID bit values of the STAs are set to 1 in the TIMelement (i.e., whether or not the STAs are paged)). In FIG. 19, STAshaving AIDs corresponding to the first, third, sixth and ninth bits inthe TIM bitmap and the other STAs are all allowed to access a channel inRAW1.

RAW Slot Duration field: This field is set to T_(s1) (whereinT_(s1)=Length of PS-Poll frame+SIFS+Length of ACK frame, or Ts1=Lengthof Null Data Trigger frame+SIFS+Length of ACK frame).

RAW Slot Boundary field: This field indicates that a TXOP (ortransmission within the TXOP) is not allowed to cross a slot boundary.

With the configurations as above, RAW1 of FIG. 19 may be used for aPS-Poll or null-data trigger frame of any STA or any short controlframes.

In the example of FIG. 19, it is assumed that the GrPS or RPS IE forRAW2 indicates that only STAs satisfying the following conditions areallowed to perform channel access in RAW2.

RAW Slot AID field: This field indicates that restriction according tobit values corresponding to AIDs of STAs in a TIM element is not applied(namely, channel access of all STAs is allowed in RAW2 regardless ofwhether or not the AID bit values of the STAs are set to 1 in the TIMelement (i.e., whether or not the STAs are paged)). In FIG. 19, STAshaving AIDs corresponding to the first, third, sixth and ninth bits inthe TIM bitmap and the other STAs are all allowed to access a channel inRAW2.

RAW Slot Duration field: This field is set to T_(s2) (whereinT_(s2)≧Length of data frame+SIFS+Length of ACK frame).

RAW Slot Boundary field: This field indicates that a TXOP (ortransmission within the TXOP) is not allowed to cross a slot boundary.

With the configurations as above, RAW2 of FIG. 19 may be used by the APor any STA to transmit a data frame to any STA or the AP.

Multicast/Broadcast Transmission Slot

When a RAW is divided into one or more time slot, the first one or moreslots or the last one or more slots in the RAW may be assigned formulticast or broadcast. STAs should be maintained in the awake state ina slot assigned for multicast/broadcast within the RAW.

To this end, a GrPS or RPS IE that defines parameters for a RAW and achannel access slot may further include a RAW Multicast/Broadcast SlotDuration field.

The RAW Multicast/Broadcast Slot Duration field may be used to inform anSTA group of information about the duration of allowedmulticast/broadcast medium access.

FIG. 20 illustrates multicast/broadcast slot assignment in a RAWaccording to one embodiment of the present invention.

In the example of FIG. 20, the first slot of RAW2 is assigned formulticast/broadcast, but the AP may transmit a multicast/broadcast framein the first slot. All STAs are in the awake state in the first slot.

In addition, the multicast/broadcast slot may also be used forre-configuration of slot assignment of RAWs.

For example, in FIG. 20, through a TIM element and a GrPS element (orRPS element) of a beacon frame, RAW1 and RAW2 may be set to allowchannel access only for specific STAs (e.g., paged STAs), and the slotsto be assigned to the specific STAs may be determined. For example, asdescribed above, an STA having AID1 may be assigned to the first slot,another STA having AIDS may be assigned to the second slot, another STAhaving AID6 may be assigned to the third slot, and the other STA havingAID9 may be assigned to the fourth slot.

STAs paged in RAW1 (i.e., STAs for which AID bits are set to 1 in theTIM bitmap of a beacon frame) may make a request to the AP fortransmission of a downlink frame buffered in the AP by transmitting aPS-Poll frame or trigger frame.

Herein, it is assumed that the STA having AID6 is assigned with thethird slot of RAW1, but it fails to switch from the doze state to theawake state on a slot boundary from which the third slot begins and thusfails to transmit a PS-Poll frame or trigger frame in the third slot, asshown in FIG. 20.

The AP has assigned a slot (e.g., the third slot) in RAW2 to transmit adownlink frame to the STA having AID6. However, since the AP has failedto receive a PS-Poll/trigger frame from the STA having AID6 in RAW1, theAP can expect that the STA will fail to transmit the PS-Poll/triggerframe in the slot of the RAW2 if slot assignment to the STA having AID6is left unchanged. Accordingly, the AP needs to collect the slotassigned for the STA having AID6.

To this end, the AP may transmit a RAW announcement frame in the firstslot of RAW2 which is assigned as the multicast/broadcast slot. The RAWannouncement frame includes a GrPS IE (or an RPS IE). That is, the APmay renew the configurations (e.g., RAW duration, RAW slot duration,slot assignment, etc.) of the next RAW (i.e., RAW2) based on whether ornot PS-Poll/trigger frames are received from the STAs in RAW1. That is,information about resource allocation in the RAW may be transmitted eventhrough a frame other than the beacon frame at the beginning of the RAW.

In this case, slot assignment for the STA is determined based on theslots other than the multicast/broadcast slot among all the assignableslots (i.e., all the slots included in the RAW). For example, in FIG.20, slot assignment of the three slots (i.e., the second, third andfourth slots) to the STA may be determined with the first slot of RAW2excluded from slot assignment for the STA. The slot assignmentinformation may be included in the RAW announcement frame (i.e., theframe containing information about resource allocation in the RAW) atthe beginning of the RAW, and a slot assignment scheme may be determinedas in the case of the aforementioned scheme.

FIG. 21 illustrates multicast/broadcast slot assignment in a RAWaccording to another embodiment of the present invention.

While the example of FIG. 20 assumes that the multicast/broadcast slotis always positioned at the beginning of a RAW, FIG. 21 illustrates acase in which the multicast/broadcast slot may be the first slot of theRAW or another slot. In the example of FIG. 21, the multicast/broadcastslot is positioned at the last part of the RAW. In this case, when theGrPS IE (or RPS IE) defines a RAW multicast/broadcast slot durationfield, a field containing information indicating the position of themulticast/broadcast slot (i.e., RAW Multicast/Broadcast Slot Offset) maybe included.

For example, if an N-th slot of RAW1 is assigned as themulticast/broadcast slot, the RAW Multicast/Broadcast Slot Offset fieldmay be set to N. If the multicast/broadcast slot is positioned at thebeginning of the RAW, the RAW Multicast/Broadcast Slot Offset field maybe set to 0. If the multicast/broadcast slot is positioned at the lastpart of the RAW, the RAW Multicast/Broadcast Slot Offset field may beset to 255.

FIG. 22 illustrates a channel access method according to one embodimentof the present invention.

In step S2210, RAW information from a first STA (e.g., an AP) may bereceived by a second STA (e.g., a non-AP STA). The RAW information maybe a GrPS element or RPS element described above and may be transmittedthrough a beacon frame.

In step S2220, based on the RAW information, the second STA maydetermine whether it belongs to a group for which channel access isallowed in a RAW, and determine the position and length in time (i.e.,the RAW start time and duration) of the RAW in which channel access ofthe second STA is allowed, and the position and length of a slot of theRAW in which channel access of the second STA is allowed. The second STAmay also determine a channel access method depending on whether or nottransmission crossing a slot boundary is allowed when transmission isperformed within the RAW by acquiring a TXOP, and whether or not onlypaged STAs are allowed to perform channel access.

In step S2230, the second STA may attempt channel access. That is, thesecond STA may access a channel based on EDCA (i.e., in a contentionmanner).

In implementing the channel access method described with reference toFIG. 22, details of the various embodiments of the present inventiondescribed above may be independently applied or two or more embodimentsmay be simultaneously applied.

FIG. 23 is a block diagram illustrating a radio frequency deviceaccording to one embodiment of the present invention.

An AP 10 may include a processor 11, a memory 12, and a transceiver 13.An STA 20 may include a processor 21, a memory 22, and a transceiver 23.The transceivers 13 and 23 may transmit/receive a radio frequency signaland implement a physical layer according to an IEEE 802 system. Theprocessors 11 and 21 may be connected to the transceivers 13 and 21 toimplement a physical layer and/or a MAC layer according to an IEEE 802system. The processors 11 and 21 may be configured to perform variousoperations according to the various embodiments of the present inventiondescribed above. In addition, modules to perform operations of an AP andan STA according to the various embodiments of the present inventiondescribed above may be stored in the memories 12 and 22 and executed bythe processors 11 and 21. The memories 12 and 22 may be contained in theprocessors 11 and 21 or may be installed at the exterior of theprocessors 11 and 21 and connected to the processors 11 and 21 by awell-known means.

Constituents of the AP and the STA may be implemented such that detailsof the various embodiments of the present invention described above areindependently applied or two or more embodiments are simultaneouslyapplied. For clarity, redundant descriptions have been omitted.

The embodiments of the present invention described above may beimplemented by various means. For example, the embodiments of thepresent invention may be implemented by hardware, firmware, software, ora combination thereof.

When implemented by hardware, a method according to embodiments of thepresent invention may be implemented by one or more ASICs (applicationspecific integrated circuits), DSPs (digital signal processors), DSPDs(digital signal processing devices), PLDs (programmable logic devices),FPGAs (field programmable gate arrays), processors, controllers,microcontrollers, microprocessors, and the like.

When embodied in firmware or software, a method according to theembodiments of the present invention may be implemented in the form of amodule, a procedure, a function, or the like which performs thefunctions or operations described above. Software code may be stored ina memory unit and executed by the processor. The memory unit may bedisposed inside or outside the processor to transceive data with theprocessor through various well-known means.

Detailed descriptions of preferred embodiments of the present inventionhave been given to allow those skilled in the art to implement andpractice the present invention. Although descriptions have been given ofthe preferred embodiments of the present invention, it will be apparentto those skilled in the art that various modifications and variationscan be made in the present invention without departing from the spiritand scope of the present invention. Thus, the present invention is notintended to be limited to the embodiments described herein, but isintended to have the widest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

As described above, various embodiments of the present invention havebeen described through examples applied to an IEEE 802.11 system, butthey may also be applied to various wireless communication systems inthe same manner

1. A method for performing channel access by a station (STA) in awireless communication system, comprising: receiving a first framecontaining a traffic indication map (TIM) element and a restrictedaccess window (RAW) parameter set (RPS) element, the RPS elementincluding a RAW assignment field indicating a restricted access window(RAW) at which the STA is allowed to perform the channel access;receiving, at a beginning of the RAW, a second frame that assigns one ormore RAW slots within in the RAW; and performing channel access based ona RAW slot for the STA among the one or more RAW slots of the RAW,wherein the RAW assignment field contains first information indicatingwhether a transmission across a RAW slot boundary is allowed to the STAand second information indicating whether channel access within the RAWis allowed to a paged STA only.
 2. The method of claim 1, wherein thepaged STA is a STA for which an association identifier (AID) bit is setto ‘1’ in a TIM bitmap of the TIM element.
 3. The method of claim 1,wherein the second frame is received from an access point (AP) in amanner of broadcasting.
 4. The method of claim 1, wherein the RAW slotfor the STA is determined based on a partial association identifier(PAID).
 5. The method of claim 1, wherein the RAW slot for the STA isdetermined based on a group identifier (GID).
 6. The method of claim 1,wherein the second frame including information on a RAW duration and oneof a group identifier (GID) and a partial association identifier (PAID).7. The method of claim 1, wherein the RAW slot for the STA is determinedbased on a modulo operation where a divisor is set to a total number ofthe one or more RAW slots.
 8. The method of claim 7, wherein a dividendof the modulo operation is set based on an association identifier (AID)of the STA.
 9. The method of claim 7, wherein a dividend of the modulooperation is set based on an association identifier (AID) of the STA.10. The method of claim 1, wherein the first frame is a beacon frame,and the performing channel access comprises transmitting a power save(PS)-Poll frame.
 11. A station (STA) performing channel access in awireless communication system, comprising: a receiver to receive a firstframe containing a traffic indication map (TIM) element and a restrictedaccess window (RAW) parameter set (RPS) element, the RPS elementincluding a RAW assignment field indicating a restricted access window(RAW) at which the STA is allowed to perform the channel access, and toreceive, at a beginning of the RAW, a second frame that assigns one ormore RAW slots within in the RAW; and a processor to perform channelaccess based on a RAW slot for the STA among the one or more RAW slotsof the RAW, wherein the RAW assignment field contains first informationindicating whether a transmission across a RAW slot boundary is allowedto the STA and second information indicating whether channel accesswithin the RAW is allowed to a paged STA only.
 12. The STA of claim 11,wherein the paged STA is a STA for which an association identifier (AID)bit is set to ‘1’ in a TIM bitmap of the TIM element.
 13. The STA ofclaim 11, wherein the second frame is received from an access point (AP)in a manner of broadcasting.
 14. The STA of claim 11, wherein the RAWslot for the STA is determined based on a partial association identifier(PAID).
 15. The STA of claim 11, wherein the RAW slot for the STA isdetermined based on a group identifier (GID).
 16. The STA of claim 11,wherein the second frame including information on a RAW duration and oneof a group identifier (GID) and a partial association identifier (PAID).17. A method for supporting channel access of a station (STA) in awireless communication system, the method performed by an access point(AP) and comprising: transmitting a first frame containing a trafficindication map (TIM) element and a restricted access window (RAW)parameter set (RPS) element, the RPS element including a RAW assignmentfield indicating a restricted access window (RAW) at which the STA isallowed to perform the channel access; and transmitting, at a beginningof the RAW, a second frame that assigns one or more RAW slots within inthe RAW, wherein the AP controls the STA to perform channel access basedon a RAW slot for the STA among the one or more RAW slots of the RAW,and wherein the RAW assignment field contains first informationindicating whether a transmission across a RAW slot boundary is allowedto the STA and second information indicating whether channel accesswithin the RAW is allowed to a paged STA only.
 18. An access point (AP)supporting channel access of a station (STA) in a wireless communicationsystem, comprising: a transmitter to transmit a first frame containing atraffic indication map (TIM) element and a restricted access window(RAW) parameter set (RPS) element, the RPS element including a RAWassignment field indicating a restricted access window (RAW) at whichthe STA is allowed to perform the channel access; and to transmit, at abeginning of the RAW, a second frame that assigns one or more RAW slotswithin in the RAW; and a processor to control the STA to perform channelaccess based on a RAW slot for the STA among the one or more RAW slotsof the RAW, and wherein the RAW assignment field contains firstinformation indicating whether a transmission across a RAW slot boundaryis allowed to the STA and second information indicating whether channelaccess within the RAW is allowed to a paged STA only.